The compounds 2-amino-5-nitropyridine, 2-amino-5-nitro-1,3-pyrimidine, and N-nitro-4-amino-1-hydropyridinium nitrate have two-dimensional layer structures with the molecules in each layer held together by hydrogen bonds, Although there are strong electrostatic interactions within each laver between the molecules or ions, the dispersion interaction is the dominant interlayer interaction and the driving force to decrease the interlayer distance. The interlayer distances of 2-amino-5-nitropyridine and 2-amino-5-nitro-1,3-pyrimidine crystals are only 3.04 and 3.07 Angstrom, respectively, which are considerably less than In graphite (3.35 Angstrom). We find that the molecules are able to pack so as to avoid C-C contacts between layers and that the separation of dir layers is determined by the size of the N atoms, which are smaller. The O atoms of neighboring nitrate anions in the N-nitro-4-amino-1-hydropyridinium nitrate crystal are in contact (separation 2.80 Angstrom), but the repulsion between them is overcome by the strong attraction between the anions and cations.